In Luke 9 and 10, the disciples were sent out with nothing so they would learn God would provide for their needs. In Luke 22 they show they miss this correlation Jesus makes: Use all God has provided, even the “shirt on your back,” for the difficult spiritual battle before you!
Then Jesus asked them, “When I sent you without purse, bag or sandals, did you lack anything?” [cf. Luke 9:1-6, 10:1-24] “Nothing,” they answered. He said to them, “But now if you have a purse, take it, and also a bag; and if you don’t have a sword, sell your cloak and buy one. It is written: ‘And he was numbered with the transgressors’; and I tell you that this must be fulfilled in me. Yes, what is written about me is reaching its fulfillment.” The disciples said, “See, Lord, here are two swords.” “That’s enough!” he replied. Luke 22:35-38
“Jesus muses over the good old days when he was able to send the disciples out and they received hospitality…When they go out on mission again, they can expect only trouble…The sword represents the opposite of peace. They will be targets, but Jesus does not authorize them to retaliate. The command to buy swords is akin to saying: “Keep your powder dry.” It is not intended to be taken literally. It means they will need every resource they have…Jesus’ command does not cause them to go buy swords. They cheerily announce that they possess two swords revealing what they have already done out of fear…”Enough” is in the singular, however, and it would not apply to the swords but must refer to something else…Jesus cuts off the conversation, as if to say, “Drop it.”
David E. Garland in Luke: Exegetical Commentary on the New Testament ed. Clinton E. Arnold (Grand Rapids: Zondervan, 2011) 870-871.